1. Field of the Invention
The present invention relates to a dielectric composition, especially suitable for microwave applications, which contains three components of BaO, TiO.sub.2 and WO.sub.3 as main components.
2. Description of the Prior Art
In recent years, dielectrics having a high dielectric constant, a low dielectric loss and a low temperature coefficient of resonant frequency have been used in the microwave region, for example, resonators and have been expanding their practical utility in microwave communication or broadcasting apparatus, for example, in car telephone systems or satellite broadcasting receivers.
Heretofore, in such applications, there have been known BaO-TiO.sub.2 system dielectric ceramics. For instance, dielectric ceramics having a composition of Ba.sub.2 Ti.sub.9 O.sub.20 are described in detail in Japanese Patent Publication No. 58-20 905. Dielectric ceramics with a composition of BaTi.sub.4 O.sub.9 have been also reported.
Further, there has been known dielectric ceramics of the composite perovskite type and Ba(Zn.sub.1/3 Ta.sub.2/3)O.sub.3 proposed in Japanese Patent Application Laid-Open No. 53-35 454 may be mentioned as a representative example of such a type.
However, the aforesaid known dielectric ceramic materials have problems in the microwave applications. For example, Ba.sub.2 Ti.sub.9 O.sub.20 can not be satisfactorily used without complicated special treatments. In the preparation of the dielectric ceramics with the Ba.sub.2 Ti.sub.9 O.sub.20 composition, calcined materials should be treated with an acid solution or after firing, the fired bodies should be treated over long periods of time in an oxygen gas atmosphere, in order to improve the dielectric properties. The Q values of the thus obtained dielectric ceramics have been reported to be on the order of 4200 at 10 GHz, wherein Q is the reciprocal of dielectric loss tan .delta., i.e., Q=1/tan .delta. [refer to "Ba.sub.2 Ti.sub.9 O.sub.20 as a Microwave Dielectric Resonator" J. K. Plourde, D. F. Linn, H. M. O'Bryan, John Thomson Jr., J. Am. Ceram. Soc., 58 (9-10) pp. 418-420 (1975) and "A New BaO-TiO.sub.2 compound with Temperature-Stable High Permittivity and Low Microwave Loss," H. M. O'Bryan, John Thomson Jr., and J. K. Plourde, J. Am. Ceram. Soc., 57 ( 10) pp. 450-53(1974)]. However, for practical use of the Ba.sub.2 Ti.sub.9 O.sub.20 dielectric ceramics as resonators, further improvements have been required in the Q values.
Further, with respect to the dielectric ceramic with the BaTi.sub.4 O.sub.9 composition, it has been reported that it has a Q value of 2500 at 11 GHz ["New Low Loss High-K Temperature-Compensated Dielectric for Microwave Application", D. J. Masse, et al., Proceedings of the IEEE 59 (11), pp. 1628-29 (1971)] and a further improved Q value has been required. Further, in the use for dielectric resonators, the BaTi.sub.4 O.sub.9 dielectric ceramic has an unfavorably large temperature coefficient of resonant frequency, .tau..sub.f, of +15 to 20 ppm/.degree.C.
The perovskite type dielectric ceramics need the use of large quantities of expensive Nb.sub.2 O.sub.5 and Ta.sub.2 O.sub.5 and thereby the production cost is increased.